[Electrical force] 2 balls hanging from ceiling, angled

[sea333]

sqrt(4a^2sin^2(10)) = 2*a*sin10

How do you get from 1 - cos^2(10) - sin^2(10) to 2 sin^2(10) ?

1 - cos^2(10) + sin^2(10) = 2sin^10 because cos^2(10) + sin^2(10) = 1